Centrifugal pumps are the most widely used pumps in the world. While there are many variations and configurations, with reference to FIG. 1 all such pumps 100 include an impellor 106 mounted on a shaft 108 that rotates the impellor 106 continuously within a housing 116. The impellor 106 includes a plurality of “blades” or “vanes” 110 extending outward from a hub and a central “eye” of the impellor. Process fluid 102 entering through an inlet 118 to the eye is caused to rotate by the impellor blades 110, and is driven by centrifugal force toward the perimeter of the impellor 106, which is in fluid communication with an outlet 104 through which the process fluid exits the pump 100.
FIG. 1 illustrates a single-stage centrifugal pump. However the discussion herein and the disclosure of the invention infra will be understood by one of skill in the art to also apply to stages of a multi-stage pump, unless the context requires otherwise.
Impellors 106 vary in the number and shape of the blades 110. For some impellors 106 the blades 110 are free-standing. However, for larger pumps it is often desirable to include a rear shroud 112, and possibly also a front shroud 114, which support the blades 110 and also reduce leakage around the blades. An impellor 106 having free-standing blades is called an “open” impellor. An impellor 106 having only a rear shroud 112 is called “semi-open” or “semi-closed” impellor 106, while an impellor having both rear 112 and front 114 shrouds is referred to as a “closed” impellor. FIG. 1 is a cut-away perspective view of a centrifugal pump 100 having a closed impellor 106.
FIG. 2 is a front perspective view of a closed impellor 106. The leading edges 202 of the blades 110 can be seen through the eye near the hub 200 of the impellor 106. The trailing edges 204 of the blades 110 are visible between the shrouds 112, 114 near the outer edge of the impellor 106.
FIG. 3A is a cross-sectional view of the impellor 106 of FIG. 2, enclosed within a single-stage pump housing 116. Wear rings 300, 302 inhibit process fluid from leaking into or out of a chamber 304 located behind the rear shroud 112. However, some process fluid leaks past the wear rings 300, 302 and fills a cavity 304 located behind the rear shroud 112. The fluid filling the cavity 304 is therefore sometimes referred to as “leakage” fluid. Frictional drag causes the leakage fluid in contact with the back of the rear shroud 112 to rotate approximately at the speed of the impellor 106, while the leakage fluid that is in contact with the housing 116 on the other side of the cavity 304 is almost static. Shear forces cause the leakage fluid in the center of the cavity 304 to rotate at a speed that is less than the impellor speed, so that the average rotational speed of the leakage fluid in the cavity 304 is comparable to one half of the impellor speed.
In contrast, process fluid located near the front surface of the rear shroud 112 is rotated by the blades 110 at approximately the speed of the impellor 106. The “static” pressure of the fluid in front of the rear shroud 112 is the process fluid inlet pressure, while the static pressure of the leakage fluid is comparable to the higher outlet pressure. The actual pressures are reduced in each case, because according to well-known fluid dynamic principles the pressure of a fluid is reduced in proportion to its velocity. Hence, because the leakage fluid is rotating more slowly than the fluid in front of the rear shroud 112, the actual pressure of the fluid immediately in front of the rear shroud 112 in the region of the eye is considerably lower than the actual pressure of the leakage fluid filling the cavity 304 directly behind the rear shroud 112. The result of the difference in static pressures as well as the difference in fluid rotation rates is an axial thrust applied to the impellor 106, which is labeled “F” in FIG. 3. This thrusting force must be opposed and withstood by the bearings (not shown) that support the pump shaft 108.
FIG. 3B is a cross-sectional view of two stages of a multi-stage pump. In this example, the leakage past the shaft seal 302 eventually flows from the rear cavity 304 back into the primary flow 102 entering the first stage. Each of the impellors 106, 306 is subject to axial thrust, as described above, such that the shaft bearings are subject to the combined sum of the axial thrusts of all the stages.
In many instances, it is desirable to reduce the axial thrust, so as to reduce the demands placed on the support bearings, and to prolong the life of the support bearings. With reference to FIG. 4, one approach is to include “balancing holes” 400 that penetrate the rear shroud 112 near the hub 200. The balancing holes allow leakage fluid to flow from the rear cavity 304 into the eye, thereby “balancing” the fluid pressures on either side of the rear shroud 112 and reducing or eliminating the axial thrust. FIG. 5 is a front perspective view of the central region of the impellor 106 of FIG. 4.
While balancing holes are effective in reducing axial thrust, they also inevitably cause a loss of pump efficiency. As can be seen in FIG. 4, the flow of leakage fluid 402 through the balancing holes 400 is in nearly direct opposition to the fluid flowing into the eye. Accordingly, the flow of leakage fluid significantly perturbs and interferes with the primary flow 102 of the process fluid, thereby significantly reducing the efficiency of the centrifugal pump.
What is needed, therefore, is a centrifugal pump impellor having balancing holes that reduce or eliminate axial thrust while minimizing the loss of pump efficiency.